The background of the invention will be discussed in two parts:
1. Field of the Invention
This invention relates to both optics and lasers. More particularly, it relates to a method and apparatus for improving the maximum power which can be transmitted through transmissive optics.
2. Description of the Prior Art
CO.sub.2 lasers in particular and other high power lasers in general have had a problem with transmissive optics at high power levels. When high power laser beams are passed through transparent materials, there is a limit to the power that can be transmitted without causing either a distortion to the transmitted laser beam or else a destruction of the transparent optics. In some cases, this limit is associated with a high power density causing a fundamental change in the optical material such that the absorption of the material increases. That type of limit associated with power density is rare and is not addressed here. The most common type of problem encountered in passing high power laser beams through transparent materials is non uniform heating of the transparent material. This, in turn, results in a build up of stresses in the material which can both distort the laser beam and also cause the transparent material to fracture. When a material experiences this type of limit, a particular material can have a figure of merit which relates to the total power handling capability of that material. For example, zinc selenide (a transparent material for use with high power CO.sub. 2 lasers) typically approaches a thermal limit when the power transmitted through the zinc selenide exceeds 2500 watts. This number depends on various factors including the coatings that are on the material and the distribution of the laser beam. For example, a lens used with a high power CO.sub.2 laser will typically have the heat removed by radial thermal conduction to a heat sink around the edge of the lens. In this case, a metal ring may make good thermal contact with the edge of the lens using an indium bond for thermal conduction. In some cases, air is also blown on the surface of the lens to produce a small amount of face cooling. The primary heat removal path is through the edge of the lens, yet the laser beam primarily passes through the center of the lens. Therefore, there is heating of the lens near the center resulting in a hoop stress built up in the lens because of the temperature profile described. This hoop stress can result either in a distortion of the lens or else in the extreme case, fracturing of the lens.
Laser metal working equipment, for example, can be quite expensive. Most of the cost is associated not with the laser but instead with the other mechanical and electronic components required to direct the laser to the proper location. The productivity of the entire piece of equipment is limited by the cutting speed of the laser. The maximum practical laser power, in turn, is limited by the power which can be transmitted through the lens. This is to say that most of these laser cutting machines are limited to about 2500 watts because this is the limit of the power handling capability of the lens. (Focusing mirrors can be used but the lens is more desirable because the lens also serves as a barrier for gas jet assisted cutting.) If it were possible to increase the power handling capability of the lens, then more powerful lasers could be used and the production rate of the entire machine could be increased. The invention described herein relates to a new design for transmissive optics used with high power lasers, such as lenses, windows, and output couplers. The description which follows will use as an example a lens system made of zinc selenide and used with a high power CO.sub.2 laser. However, it is to be understood that the invention described herein relates to windows and output couplers as well as other materials and lasers.